Laser welding is widely used in titanium alloy welding due to its high energy density, small heat affected zone, and rapid processing ability. However, problems with laser welding, such as deformation and cracking caused by residual stress, need to be resolved. In this paper, the residual stress in laser welding of TC4 titanium alloy was studied using an ultrasonic laser. The residual stress in titanium alloy plates is considered a plane stress state. A pre-stress loading method is proposed and acoustoelastic coefficients are obtained. Based on the known acoustoelastic coefficients, the transverse and longitudinal residual stresses in laser welding are measured using an ultrasonic laser. The results show that longitudinal residual stress is greater than the transverse stress. The distribution regularity of the residual stress is similar to normal welding, but the tensile stress zone is much narrower. Then, the influence of heat input and welding speed on residual stress is discussed. With increasing heat input, the welding zone widens, and the peak value of the residual stress increases. A higher welding speed should be chosen when the welding power is constant. This research has important significance for the measurement and control of residual stress in the laser welding process.

Vacuum induction melting gas atomization (VIGA) has evolved as an important production technique of superalloy powders used in additive manufacturing. However, the development of powder preparation techniques is limited because the crushing process of gas-atomized metal melt is difficult to characterize by conventional experimental methods. Herein, we report the application of computational fluid dynamics to simulate the breaking behavior of droplets in the process of preparing nickel-based superalloy powders by VIGA, as well as the results on the effect of gas pressure on the atomization process and powder particle size distribution of metal melt. In the process of primary atomization, the crushing morphology of superalloy melt shows an alternate transformation of umbrella shapes and inverted mushroom cloud shapes, and with the increase in atomization pressure, the disorder of the two-phase flow field increases, which is conducive to sufficient breakage of the melt. Most importantly, in the process of secondary atomization and with the increasing atomization pressure, the particle size distribution becomes narrower, the median particle diameter and average particle size decrease, and the decreasing trend of the particle size increases gradually. The simulation results are compliant with the performed nickel-based superalloy powder preparation tests. This study provides insight into the production and process optimization of superalloy powder prepared by the VIGA method.

This paper considers a single-machine scheduling problem with past-sequence-dependent delivery times and the truncated sum-of-processing-times-based learning effect. The goal is to minimize the total costs that comprise the number of early jobs, the number of tardy jobs and due date. The due date is a decision variable. There will be corresponding penalties for jobs that are not completed on time. Under the common due date, slack due date and different due date, we prove that these problems are polynomial time solvable. Three polynomial time algorithms are proposed to obtain the optimal sequence.